Conventionally, a screw is widely used as a joining means for members. As a common means for tightening such a screw, conventionally, a female recess such as a plus (+)-shaped “cross recess” or a minus (−)-shaped “slot” is formed in the top portion of the screw, so that by fitting therein the distal end of a screw driving device, such as a correspondingly formed screwdriver, and turning, running torque for fastening the screw is applied to the screw.
Also, in addition to the method that forms such a “cross recess” or a “slot,” there is also known one that forms a recess of a special shape such as a hook-shaped “claw” in a circumferential side portion of the head portion, and so by fitting in this claw a special screwdriver having a corresponding distal end shape, fastening torque is applied to the screw.
On the other hand, as an actuator capable of exerting high running torque, there is known an ultrasonic motor that can obtain high stationary torque particularly when the rotor is stationary, as disclosed for example in Non-patent Document 1 given below. Non-patent Document 1: Kentaro Nakamura, Minoru Kurosawa, and Sadayuki Ueha: “Characteristics of a Hybrid Transducer-Type Ultrasonic Motor”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 38, No. 3, May 1991, p.188-193. Non-patent Document 2: Hiroshi Kawano and Hideyuki Ando: “Development of Piezo Electric Screwdriver for a Recessless Screw”, Proceedings of the 2004 Conference on Robotics and Mechatronics, Japan Society of the Mechanical Engineers, June 2004.